i) Field of the Invention
The present invention relates to a method for producing a thermal head, and more particularly to a method for producing a thermal head with a uniforming step of resistance values of heating resistors.
ii) Description of the Related Arts
In general, as shown in FIG. 1, a conventional thin film thermal head includes a ceramic substrate 1, a glaze layer 2 formed on the ceramic substrate 1 and a heating resistor 3 formed on the glaze layer 2 and the heating resistor 3 is composed of a thin film resistor material of a mixed composition of a high melting point metal and an insulating material. Then, a first pattern conductor 4 for providing a common electrode and a second pattern conductor 5 for providing separate electrodes are formed on the heating resistor 3 so that a part of the heating resistor 3 may be exposed in a band form, and the first and second pattern conductors 4 and 5 and the exposed part of the heating resistor 3 are covered by a protective film 6 to produce the thin film thermal head. Usually, in order to obtain a predetermined dot number, a plurality of the thin film thermal heads are arranged in a printing scanning direction.
In the thin film thermal head, a dot resistance value is determined by a dot size and a sheet resistance of a resistor film when the resistor film composed of the thin film resistor material of the mixed composition of the high melting point metal and the insulating material is prepared. The dispersion of resistance values of heating resistors in the thermal head can constitute a factor for deteriorating the printing quality and is caused by the dispersion of the sheet resistances of the resistance films and the dispersion of the dot sizes caused by the condition variation of an etching or the like in a photolithographing step.
On the other hand, the primary factor for the dispersion of average resistance values of the heating resistors in each head is due to the dispersion of the sheet resistances of the resistor films produced by sputtering or the like by using a sintered mixture target.
In such a case, by reducing the dispersion of the resistance values, a variety of advantages can be obtained as follows. For example, the printing quality can be improved, and an applied voltage adjusting is not required depending on the resistance values of the heating resistors in each head when a plurality of heads are mounted on a printer. However, there are many technical difficulties for reducing the dispersion of the resistance values.
A variety of methods concerning a trimming process of a thin film resistors have been proposed in order to suppress the dispersion of the resistance values. However, in all of these methods, the shape of the resistor is changed by a laser, a blaster or the like, but this method is not suitable for the thermal head. Hence, it is necessary to suppress the dispersion of the resistance values of the heating resistors without performing the trimming processing. In this case, it is required to strictly control the necessary conditions such as sputtering conditions, sputtering materials, photolithographing conditions and the like in performing various processings in the thermal head producing method. If the conditions are varied a little bit, the dispersion of the resistance values will be caused.
On the other hand, as described above, the thin film resistor material of the mixed composition of the high melting point metal and the insulating material is used in order to realize a high resistivity of the heating resistor. Usually, this thin film resistor material is used for forming a resistor film by a sputtering method using a mixed sintered target. However, in the resistor film of a polygene system material composed of the thin film resistor material, a structural defect of the film is apt to occur and the resistivity of the resistor film is liable to be changed with the variation of the sputtering conditions. That is, as the degree of the vacuum of the sputtering is raised, the resistivity is increased. Hence, if the sputtering conditions are not strictly controlled, the dispersion of the resistance values of the heating resistors is caused. Accordingly, the calorific value is different in the resistors and a density difference is caused in the dot printing, which becomes the primary factor for deteriorating the printing quality.
As described above, conventionally, the problems appearing in the heating resistors composed of the thin film resistor material are summarized as follows.
(1) The strict control of the sputtering conditions is required. That is, the variation of the conditions causes the variation of the resistivity of the heating resistors, and as a result, the dispersion of the resistance values of the resistors is caused after the production of the resistors. This can be the factor for causing the dispersion of the printing quality or grade.
(2) The resistance value of the heating resistor in the thermal head is reduced after the printing. This also causes the dispersion of the printing quality or grade. For example, when a pulse application of the dots printed many times at the initial time as the thermal head and the dots printed not many times is performed, since there is a difference among the resistance values of the resistors constituting the dots, the dispersion is caused in the printing density and the dot size. The dispersion of the printing quality or grade is particularly remarkable in a head for a color printer which strictly requires uniformity of the resistance values per dot unit.